1. Field of the Invention
The present invention relates to a wireless tag reader/writer that transmits and receives a signal to or from a wireless tag, and more particularly, to a wireless tag reader/writer capable of changing an output power of a radio wave to be emitted.
2. Description of the Related Art
In case a small number of wireless tags are read out or written at a time, an output power of a radio wave emitted from a wireless tag reader/writer does not need to be large. However, in case a large number of wireless tags are read out or written at a time, the output power of the radio wave emitted from the wireless tag reader/writer needs to be large. However, when the output power of the radio wave emitted from the wireless tag reader/writer is made large, interference with a radio wave emitted from another wireless tag reader/writer occurs, and wireless tags may not be appropriately read out or written. For this reason, an output power of a radio wave emitted from a wireless tag reader/writer needs to be controlled.
With respect to the above point, there has been suggested a technique for communicating with wireless tags by emitting a radio wave in both a short and a long transmission intervals without setting the wireless tags in a sleep mode and without maximizing a transmission output power of the radio wave (for example, see Jpn. Pat. Appln. Laid-Open Publication No. 2005-535968). In addition, there has been suggested a technique in which a wireless tag reader/writer is controlled to display a success rate of communication with wireless tags (for example, see Jpn. Pat. Appln. Laid-Open Publication No. 2005-235180).
However, according to the technique described in Jpn. Pat. Appln. Laid-Open Publication No. 2005-535968, an output power of a radio wave is not made high. On the other hand, in case wireless tags are of a passive type that receives a radio wave to generate power, an amount of power necessary for activating one wireless tag is predetermined. Therefore, according to this technique, there has been a problem that an amount of power is insufficient for reading out and writing a large number of wireless tags.
In addition, in the above-described technique, a radio wave having a long transmission interval is transmitted. On the other hand, a frequency band that can be used is limited. Therefore, according to the technique described in the above Publication, there has been a problem that a frequency band cannot be used efficiently. With respect to this point, depending on a frequency band of a transmitted radio wave, executive authorities may limit continuous transmission of a radio wave for a certain period of time (for example, Minister for Public Management, Home Affairs, Posts and Telecommunications Order Subparagraph 49 based on Radio Facility Regulation Article 49-9 based on Radio Law, and Minister for Public Management, Home Affairs, Posts and Telecommunications Order Subparagraph 385 based on the same Regulation Article 49-14). In this case, there has been a problem that the technique described in this Publication cannot be adopted.
In addition, the technique described in Jpn. Pat. Appln. Laid-Open Publication No. 2005-235180 merely displays a success rate of communication with wireless tags, and there has been a problem that a specific technique for reading out and writing a large number of wireless tags existing densely is not suggested.